Panama plans for Bajo de Mina

1 September 2004

Eduardo Castell, Carlos Noguera and Elkin Darío Ospina present preliminary results from a feasibility study currently in progress at Panama’s Bajo de Mina hydro power project

The Bajo de Mina hydroelectric project is located on the Chiriqui Viejo river in the northwest region of the Republic of Panama, in the province of Chiriqui, Renacimiento district, Plaza Caisán area, very close to the border with Costa Rica. This area has very high hydroelectric potential, especially the Chiriqui Viejo river which is the most extensive in the country.

The area was initially studied by the Panamanian electric company in the 1980s and 1990s. La Mina Hydro-Power Corporation – the current owner of the project – identified it in 1998–1999 and prepared a number of preliminary surveys, from which a feasibility revision dated October 2000 commissioned to KGS International of Canada stands out. Recently, HMV Ingenieros Ltda (formerly Hidroestudios S.A.) of Colombia was hired to perform a feasibility study, with work ongoing.

Bajo de Mina is an edge of water project that exploits a 122.5m raw downfall. The project outline includes a 30m high dam at an elevation of 502.5m above sea level (asl). The intake will be on the left bank, leading to a 3815m long tunnel to the power house in an alluvial terrace formed by the same river, at an elevation of 380m asl. The dam forms a reservoir that allows an acceptable daily regulation; the river carries an average 56.38m3/sec enabling an installed capacity of 52.32MW and a reliable annual output of 259GWh.

The project will be connected to the Panamanian high voltage grid. This generation project would operate at 13.8kV and can provide a firm guaranteed power of 28MW. Studies showed that a 115kV, 30km simple circuit line was the best technical and economic alternative for connecting the Bajo de Mina power plant to the Panamanian grid over the 115kV Progreso substation busbar. An elevator 13.8/115kV transformer is also required at the plant site.

The project should be generating power by January 2008 as La Mina Hydro Power Corp. has an allocated power purchase agreement with the Panamanian Elektra company for the total firm guaranteed power and its associated energy by that date.

Geology

The geology is characterised by recent volcanic activity in the mountain range. Tertiary poorly consolidated sedimentary rocks overlying basalts have been lifted by tectonic events to the elevation of the project area, and more recent volcanic materials have been deposited on top of these rocks. Seismic activity in the region is high, with intense earthquakes likely.

Good physical and mechanical property sandstones overlying basalts are found at the foundation level dam axis. These rocks are believed to have good strength and permeability characteristics for the foundation of a roller compacted concrete (RCC) dam. To reduce rock permeability and seal diaclases, consolidation and drainage grouting were designed.

Based on on-site observations, gathered surface information and the geological survey, it is believed that the tunnel will cross fairly hard tertiary but highly erodable sandstones in its first 1000m. Form this point on, it will go through a lithologic package with high and regular alteration levels, sandstones and varied thickness shales, with conglomerate insertions.

The water intake gate will be excavated on an almost vertical slope of sandstone outcrops that starts at river level and elevates to a height of 520m asl. This is a 45m thick wall comprised of semi-hard tertiary sandstones allowing supports that will guarantee concrete structure stability.

Hydrological studies

Morphometric characteristics of the basin have been defined; sub-basin division performed and areas measured; and lengths of the main riverbeds and slopes have been calculated. Information from three water discharge stations and 12 rainfall stations was gathered and analysed, with an average of 30 year history information. Water yield conditions for the project were obtained in order to determine potency and energy characterisation for the economic appraisal.

To define the firm power for the project, a water volume of 10.93m3/sec with 95% feasibility of occurrence was considered. Based on extreme flows recorded and extreme precipitation information, the flood hydrographs with diverse return period were evaluated to size diversion tunnel works and spillway capacity. The peak flow obtained varied between 375m3/sec for mean annual flood and 1515m3/sec for a 1000-year flood.

Explorations

As part of the project exploration for the probable tunnel outline, dam axis and power house area, a geophysical survey was performed by the seismic refraction technique, supplemented with drillings, to obtain top soil thickness, rock lithologic packages and quality. Different density alteration and/or fracturing of the lithologic packages based on the measured longitudinal wave velocity were also measured.

This indirect prospection technique is a critical supplement to the surface geological survey and the results of borings to define the geological-geotechnical model of the site. Laboratory tests will be performed on drill samples to define the mechanical properties of the materials found.

Diversion tunnel and low level discharge

The diversion tunnel – around 5m in diameter, 200m long and with a 1.56% slope – is to be located on the left bank, under and very close to the water intake. This tunnel will be excavated on rock to divert the Chiriqui river during dam construction and will operate during the life of the project as the low level discharge for sediment removal.

The first stretch of the tunnel will be curved up to the dam axis. At the tunnel intersection with the dam axis, a square section will be constructed to contain the radial control bottom discharge gate. At this intersection, a 7m diameter gates shaft will be built to an elevation of 510m asl where a gate operation shack will be located. The tunnel will be 200m long, with an outflow to the river in a straight stretch ending in a tailrace channel.

The whole tunnel will be lined in concrete, with shotcrete, bolts and steel arches used for supports during excavation. Part of the tunnel including the square section will be steel-lined to avoid erosion and damage during deviation and during bottom sediment discharge. The excavation system is to be conventional, with blasting controlled, beginning by the downstream exit portal from the dam axis.

Dam

The design studied is an RCC gravity dam, which will keep construction time short; provide safety in a high seismic activity location; allow the construction of a stepped spillway in the downstream face for flooding events from 1 to 10,000 years; and cause less environmental impact than an earth dam.

Dam height will be 30m at spillway level and 31.5m at crest and overflow level. The stepped spillway will begin at an elevation of 502m asl, and will have a useful length of 92m in the crest. The original canyon morphology has a length of 95m at 502m asl and 27m at the bottom of the river. With the planned excavation, the foundation level of the dam is to be located at approximately 472m asl. The dam axis on the selected site will be only 125m at the crest and 27m at the riverbed. The width of the crest is planned to be 5m. The dam would have a 0.8H:1V downstream slope, whereas the upstream face is to be vertical.

At 476m asl, a grouting and drainage gallery will be constructed inside the dam from where drillings will be made for contact grouting and boreholes for the curtain grouting system. Drainage drillings will also be made to dissipate hydrostatic pressures.

The dam body will be composed of 105 layers in 0.3m lifts, for a total RCC mix volume of 35,000m3. A 25mm bedding mixture will be extended between layers for a total volume of 3000m3. A concrete grout 0.4m wide strip will be incorporated in the RCC of the upstream face and will be vibrated to create a relatively watertight barrier and a good superficial finish (grout enriched vibrated RCC mix methodology.)

The design parameters for this type of dam are tension strength, heat generation, deformity capacity, strength characteristics and the shear strength of the horizontal construction joints of the RCC.

On the downstream face a stepped spillway with a slope of 0.8H:1V will be designed. As the RCC is placed in layers and it is possible to pour conventional concrete at the same time as RCC construction, this will be economical and easy.

Consolidation groutings 6m long will be made on a strip close to the dam axis on the rock foundation. Contact groutings and the curtain grouting system will be made from the drainage gallery. Both will have a separation of 2m along the gallery axis. The boreholes for the contact will penetrate at least 1m into the rock. The boreholes for permeability grout will be 18m long.

Dam instrumentation including thermocouples, piezometers, extensometers, joint displacement meters and accelerometers will be located at three elevations: 477.4m, 487m and 498.5m asl.

Spillway and dissipation pool

Based on the structural shape of the dam, the overtopping height was set at 502m asl. The spillway has a maximum discharge capacity of 1515m3/sec and it stretches along the crest of the dam. It is a concrete stepped chute, 92m long at the crest, that dissipates energy through the macro roughness of the steps.

At the base, downstream from the dam, a 30m x 27m concrete slab; a 7m long, 27m wide dissipation pool; and a 4m high spillway dike will be constructed creating a dissipation pool to deliver overflowed water in a safe way to the river and protect the dam toe.

Water intake

The water intake will be constructed on the left bank, very close to the low level discharge at an elevation of 490m asl. The proximity of the intake to the bottom discharge will promote sediment removal. This structure will have typical cleaning grids, a gate and equipment to operate it. To establish the intake structure, an excavation will be made at the highest part of the slope, forming a platform at 507m asl where maximum river water levels cannot reach. The gate operation house will be built on top of this platform.

Pressure tunnel

The pressure tunnel carrying water from the reservoir to the power house is to be 3815m long between elevations of 490m and 380m asl. The tunnel is to be 3.6m in diameter and will be totally concrete-lined. The outlet of the tunnel will have a short steel-lined section and a penstock with a trifurcation.

Having identified the rock type as sandstones, shales and medium hard highly erodable conglomerates, and considering the seismic activity of the region, it is believed that the tunnel should be lined with 0.3m thick concrete for stability and durability.

Conventional blasting for tunnel excavation with a controlled blasting plan to loosen sandstones is planned. A milling machine will also be used to cut the material if it is not too hard.

For tunnel excavation, with the conventional system, four construction fronts have been considered:

• One at the tunnel outlet, close to the power house,

• Two fronts at the construction shaft beginning at Mina Creek,

• One at the intake.

Excavating the tunnel from these four fronts will take less than 15 months.

The surge shaft has been located on a hill at the outlet of the tunnel. It is a 14m diameter, 85m high cylindrical concrete structure, with a bottom opening that links the water load tunnel with the 3.2m diameter, 71m long concrete surge shaft.

Power house

The power house is located on the surface on an alluvial terrace on the left bank of the river. This terrace is protected because the river narrows topographically creating a natural dike just upstream from the site, so the area is believed to be safe and appropriate.

The main component of the power house will be three levels high: an operation floor, a generation floor and a valve floor. In cross section, it will be a maximum of 20m high with a 372.6m asl minimum elevation, operation floor at 384.7m asl, turbine axis at 378.4m asl and the tail race channel at 380m asl.

The plant will have three generating units with 11m between axes and a 10m long assembly area. There will also be a control and command building where the control room will be located.

Each generating group will comprise a control butterfly valve, a Francis turbine and associated generator, an output cone, a bearing changing curved elbow and an intake aspiration tunnel, with an isolation output gate. This system will deliver water to a common evacuation channel to the central station which, in turn will return the waters to the natural riverbed.

Electrical equipment

The generator units will be synchronous, vertical shaft, salient poles, air-cooled, equipped with air-water heat exchangers, with continuous nominal power for a temperature increase of 75°C and synchronous speed in accordance with the alternatives. There will be a power factor of 0.9, nominal 13.8kV voltage and 60Hz. The stator and rotor will have class F insulation.

Exciter equipment for each unit will be static, brushless, and designed for local, manual, automatic and supervisor remote control.

The concept of main and back-up protection was adopted for generators, transformers and transmission lines. Protective relays will be the solid state, numeric, multifunctional type and microprocessor controlled. They will be provided with communication units that allow them to be connected in network configuration, using normalised protocols.

Appropriated power and lighting boards will be provided for maintenance and general use purposes.

The Bajo de Mina 115kV substation will be a conventional outdoor type (AIS), located adjacent to the power house. A single busbar arrangement is considered for the substation. The step-up transformer consists of a bank of three single-phase transformers with a total rated power equal to the total generation capacity. In addition, there is a spare single phase transformer if any of the units fail.

The 13.8kV switchgear will be located inside the power house and will have the adequate number of cubicles; one for each generator circuit breaker, one for the station transformer panel and another for metering.

To supply the 13.8kV/480kV, the Bajo de Mina power plant auxiliary services, two station transformer feeders were considered; one incoming main feeder from the generation medium voltage switchboard and the second from a local 13.8kV transmission line, used only as a back up when the main feeder is not available. A diesel power plant is considered for special incidents, when neither the main feeder nor the back-up feeder is available.

Each generator’s auxiliary services cubicle; 480 V–208 V/120 V auxiliary transformers that feed the miscellaneous auxiliary boards; general services like pumps, motors, and other electrical equipment associated with the power plant are fed by the 480V auxiliary services system board.

Another 13.8kV transmission line has been considered to supply the dam electrical equipment requirement through a 13.8kV/480V distribution transformer.

The system also comprises batteries and battery chargers to feed the 125Vcc and 48Vcc auxiliary services.

As a general design principle, all auxiliary supply boards include one main and one auxiliary feeder in order to obtain high reliability and availability.

The control system will provide supervision of power station equipment located in the power house, the dam and in the substation. Supervision functions will be performed from the HMI located in the control room, using the latest technology in communication control networks. The control system will acquire information from the electromechanical units and subsystems, integrating them into one single system to allow global operation and supervision of the power station from a central site.

In order to satisfy the communication services required at the hydro power station, the following systems will be implemented:

• Internal, multi mode optical fibre network into the power station between the control building / power house and dam

• External, single mode optical fibre network connecting the power station with the Progreso substation

• A telephone commutation system to supply all required necessities

Communication systems will support services for data transfer, voice/fax and voice channels for power plant operation.

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